ODL is different from the other models studied so far in that the declaration and use of keys is optional. That is, in the E/R model, entity sets require keys to distinguish members of the entity set from one another. In the relational model, where relations are sets, all attributes

As the E/R model is meant to be converted into a model such as the relational model when we put into operation the design as an actual database, ODL was initially meant to be used as the specification language for real, object-oriented DBMS's. Nevertheless ODL, like all object-

Unluckily, even when a class properties are all attributes we may have some trouble converting the class to a relation. The reason is that attributes in ODL can have complicated types such as structures, sets, bags, or lists. On the other hand, a basic principle of the relational